After you reach Humilau City, Hugh tells you to defeat the Gym Leader before you pursue the Shadow Triad.
Go through the Gym
Talk to Clyde near the entrance to get your Fresh Water. Go north onto the dock and walk to the left. Walk onto the lilypad to the left. It will float over to Ace Trainer Doyle, who has a level 47 Floatzel and a level 47 Mantine.
Use the lilypad to the right of the Ace Trainer. Notice that when it bumps into another lilypad, it stops, pushing the other lilypad in the direction that you were going.
Go up onto the dock that you stopped below. Go to the top of this dock, then hop onto the lilypad to the right. Go up to the dock above to battle Ace Trainer Mellina, who has a level 48 Alomomola.
After defeating her, go south onto the lilypad below her. Go onto the dock below you, then go to the right and up, and cross the lilypad up there. Cross the bridge, then go to the left and up. For this lilypad, enter it from above so that it moves downward. Get off of the lilypad, then enter it from the left. You can now go up to Ace Trainer Santino on the small piece of dock. He has a level 48 Walrein.
Go south across the lilypad and go right across the bridge. Use the lilypad to the right of the bridge to go up. On this dock, go up and you will find Ace Trainer Jeanne, who has a level 47 Pelipper and a level 47 Wailord. After defeating her, enter the nearby lilypad from the right side.
Go onto the dock and enter the next lilypad from below. To the right, you find Ace Trainer Sable, who has a level 47 Golduck and level 47 Starmie. After defeating her, enter the lilypad below you.
Exit from the right of the lilypad to find Ace Trainer Enzio, who has two level 46 Basculins and a level 46 Azumarill.
Now get back on the lily pad to go to the dock to the left. Get back on the same lilypad from the bottom to go upward. Go onto the dock, then enter the lower lilypad from the left. You'll end up in front of Marlon.
Marlon
Marlon has a level 49 Carracosta, level 49 Wailord, and level 51 Jellicent.
When you win, Marlon gives you the Wave Badge, which allows you to control all pokémon, including traded pokémon. He also gives you TM55 Scald. Then he dives into the water and swims away.
Ride the lilypad back to the entrance. Outside, Hugh says that before you can challenge the Pokémon League, you need to deal with Team Plasma first. Marlon shows up. He's never heard of Team Plasma, so tell him that they're bad. He'll think about that and leave, then Hugh will ask you to check Route 22 for Team Plasma. Route 22 is northwest of Humilau City.